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ΛCDM Large Scale Triumphs and Small Scale Challenges

ΛCDM has become the standard cosmological model because its
predictions agree so well with observations of the cosmic microwave background
and the large-scale structure of the universe. However ΛCDM has faced
challenges on smaller scales. Some of these challenges, including the “angular
momentum catastrophe" and the absence of density cusps in the centers of
small galaxies, may be overcome with improvements in simulation resolution and
feedback. Recent simulations appear to form realistic galaxies in agreement
with observed scaling relations. Although dark matter halos start small and
grow by accretion, the existence of a star-forming band of halo masses
naturally explains why the most massive galaxies have the oldest stars, a
phenomenon known as galactic “downsizing." The discovery of many faint
galaxies in the Local Group is consistent with the large number of subhalos
in ΛCDM simulations. There is increasing evidence for such
substructure in galaxy dark matter halos from gaps in cold stellar streams
in the Milky Way and Andromeda and from gravitational lensing flux
anomalies, with the prospect of rapidly increasing data on that from ALMA. The
“too big to fail" (TBTF) problem is the latest apparent challenge
to ΛCDM. It arose from analysis of the Aquarius and Via Lactea
very-high-resolution ΛCDM simulations of Milky-Way-mass dark matter halos. Each
simulated halo has ∼10 subhalos so massive and dense that they
would appear to be too big to fail to form lots of stars. The TBTF problem is
that none of the observed dwarf satellite galaxies of the Milky Way or
Andromeda have stars moving as fast as would be expected in these densest
subhalos. This may indicate the need for a more complex theory of dark matter –
but several recent papers have shown that subhalos in pure dark matter
simulations like Aquarius or Via Lactea are significantly modified when
baryonic effects are included, so as to solve the TBTF problem. Higher
resolution simulations are needed to verify this.